Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are now known and in commercial usage. Such permanent magnets contain as an essential magnetic phase grains of tetragonal crystals in which the proportions of iron, neodymium and boron (for example) are exemplified by the empirical formula Nd.sub.2 Fe.sub.14 B. These magnet compositions and methods for making them are described in U.S. Pat. No. 4,802,931. The grains of the magnetic phase are surrounded by a second phase that is typically neodymium-rich as compared with the essential magnetic phase. It is known that magnets based on such compositions may be prepared by rapidly solidifying a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. Magnets may be formed from these isotropic particles by practices which are known and which will be discussed further herein.
Melt spinning is an efficient method of producing rapidly solidified particles of iron-neodymium-boron compositions. The melt-spun particles, either as is or after a suitable anneal, are magnetically isotropic and have high coercivity at room temperature. They may be used to make resin bonded magnets that are magnetically isotropic. The isotropic powder has many useful applications, but there is also a need for an anisotropic powder with a coercivity of at least 1,000 Oersted at room temperature.
It is also known that iron-neodymium-boron permanent magnets can be prepared starting with cast ingots or atomized powder of suitable compositions. The ingots or powder are comminuted to form micron-size (e.g., 1 to 15 microns) powder. These particles are magnetically anisotropic. They are aligned in a suitable magnetic field, compacted into magnet bodies and sintered to form permanent magnets.
When iron-neodymium-boron ingots are pulverized, the resulting powder is magnetically anisotropic, but it has little coercivity. Similarly, if a melt is atomized by conventional atomization techniques, such powder is magnetically anisotropic but has little coercivity. It is only after such powder has been compacted and sintered that the magnets display any appreciable coercivity. Workers have attempted to pulverize such anisotropic permanent magnets in order to obtain a coercive anisotropic permanently magnetic powder. Unfortunately, however, pulverization of the permanent magnet bodies yields a powder that has little coercivity.
It is known that the melt-spun isotropic powder can be suitably hot pressed and/or hot worked and plastically deformed to form high strength anisotropic permanent magnets. This practice is described in U.S. Pat. No. 4,792,367. Such magnets have excellent magnetic properties. However, there remains a need for a magnetically anisotropic high coercivity iron-neodymium-boron type composition powder that can be magnetically aligned and molded with a suitable bonding agent to form a bonded anisotropic permanent magnet.
Accordingly, it is an object of our invention to provide magnetically anisotropic, high coercivity (e.g., greater than 1,000 Oersted at room temperature) particulate compositions based on iron, neodymium and/or praseodymium, and boron. As will be described, suitable amounts of other elements such as cobalt, nickel, aluminum, copper and the like may be added as well as suitable amounts of other rare earth metals. However, the composition of our powder is based on the essential constituents of iron, neodymium and/or praseodymium, and boron.
It is also an object of our invention to provide a method of making such magnetically anisotropic and coercive powder.